Some of the compounds of formula I, more exactly those wherein R is hydrogen or acetyl and R.sub.1 and R.sub.2 independently represent hydrogen, (C.sub.1-4)-alkyl, benzyloxy, mono- or di-(C.sub.1-3)-alkylamino-(C.sub.1-4)-alkyl, (C.sub.1-3)-alkoxy-(C.sub.1-4)-alkyl, hydroxymethyl, hydroxy-(C.sub.2-4)-alkyl, nitro, or R.sub.1 and R.sub.2 taken together with two consecutive carbon atoms of the pyridine nucleus form a benzene ring optionally substituted by one or two methyl or ethyl groups are known in the patent literature; see, for instance, U.S. Pat. No. 4,341,785 published on July 27, 1982.
Other compounds of formula I, more exactly those wherein R is hydrogen or acetyl and at least one of R.sub.1 or R.sub.2 represents halogen, cyano, mercapto, (C.sub.1-4)-alkylthio, phenylthio, carbamoyl, mono- or di-(C.sub.1-4)-alkylcarbamoyl are described in the copending Italian patent application No. 3626 A/82, now available to the public.
In these two references a process for obtaining the compounds of formula I was also described.
Briefly, the known process was carried out by reacting a molar amount of a 3-halogen rifamycin S of formula ##STR4## wherein R represented hydrogen or acetyl and halo preferably represented bromine or iodine, with from about 2 to about 8 molar equivalents of an appropriate 2-amino-pyridine of formula III.
A compound of formula ##STR5## was obtained, which was preferably isolated and characterized, and was subsequently treated with ascorbic acid in order to give the end compounds of formula I.
Notwithstanding the yields of the two cited steps, calculated on the starting compound IV, are sometimes quite good (they are comprised between about 45% and about 75%), the process described in the U.S. patent is strongly limited by the fact that the starting compound IV is not a commercial product, but has to be prepared every time starting from rifamycin S with suitable, often troublesome, processes of halogenation. This fact causes the process described either in U.S. Pat. No. 4,341,785 and in the copending Italian application 3626 A/82 to run with yields far lower than those above mentioned, thus giving the desired compounds with unsatisfactory yields from an industrial point of view.